P2836 R1 std::basic_const_iterator should follow its underlying type's convertibility

[wg21bot]

P2836R0 std::const_iterator often produces an unexpected type (Christopher Di Bella)

[brycelelbach]

@inbal2l please have ranges look at this first.

[inbal2l]

Was discussed on LEWG Mailing List.

There were 4 modifications proposed in the thread:

1. Fix const_iterator_t to:
   template
   using const_iterator_t = decltype(ranges::cbegin(declval<R&>()));
2. Fix const_iterator_t to accept arrays of unknown bound
   (addressing: https://cplusplus.github.io/LWG/issue3913)
3. Fix basic_const_iterator<int*> is not convertible to int const*
   Adding conversion operators to basic_const_iterator<I> so that it is convertible to any type that I is convertible to.
4. Proposed in P2836: Fix const_iterator<int*> isn't equivalent to int const*
   Using an associated type such as const_iterator_for and have std::vector::iterator identify that std::vector::const_iterator is its const_iterator counterpart by adding const_iterator_for as a member-alias (ABI implications to all iterators in std).

Option 1 got broad support (including from the paper author) so it will be fixed for C++23 as a library issue.
For the other options more discussion is required, therefor the paper will be scheduled for an LEWG meeting.